Syntheses and photophysical studies of new classes of luminescent isocyano rhenium(I) diimine complexes

Coordination Chemistry Reviews 256 (2012) 1546–1555

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Review

Syntheses and photophysical studies of new classes of luminescent isocyano rhenium(I) diimine complexes Chi-Chiu Ko ∗ , Apple Wai-Yi Cheung, Larry Tso-Lun Lo, Jacky Wai-Kit Siu, Chi-On Ng, Shek-Man Yiu Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Contents 1. 2.

3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1546 Synthetic methodology and characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1547 2.1. Di(isocyano) dicarbonyl rhenium diimine complexes [Re(CO)2 (CNR)2 (N–N)]+ [47] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1547 2.2. Tri(isocyano) carbonyl rhenium diimine complexes [Re(CO)(CNR)3 (N–N)]+ [48] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1548 2.3. Tetra(isocyano) rhenium diimine complexes [Re(CNR)4 (N–N)]+ [46,49] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1548 Photophysical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1548 3.1. UV–vis absorption properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1548 3.2. Emission properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1550 3.3. Emission solvatochromic behaviour of [Re(CNR)4 (N–N)]+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1552 Transient absorption spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1553 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1554 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1554 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1554

a r t i c l e

i n f o

Article history: Received 20 September 2011 Received in revised form 12 December 2011 Accepted 6 January 2012 Available online 16 January 2012 Keywords: Rhenium Isocyanide Polypyridine complexes Emission solvatochromism MLCT

a b s t r a c t The rich excited state properties associated with the long-lived triplet metal-to-ligand charge transfer excited state of rhenium(I) tricarbonyl diimine complexes have been investigated for many years. Through judicious design and functionalization of the ligands with various functional moieties, these complexes can be used for different applications. However, further modification of the physical and excited state properties with their functions being maintained are challenging. In view of this, we have designed several new classes of isocyano rhenium(I) diimine complexes with readily tunable structural features. In this review article, our recent work on the syntheses, characterizations and photophysical properties of these tunable rhenium(I) diimine luminophores are discussed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

Abbreviations: N–N, diimine ligand; phen, 1,10-phenanthroline; Ph2 phen, 4,7-diphenyl-1,10-phenanthroline; Br2 phen, 5,6-dibromo-1,10-phenanthroline; Me3 NO, trimethylamine N-oxide; bpy, 2,2 -bipyridine; t Bu2 bpy, 4,4 -di-tertbutyl-2,2 -bipyridine; (MeO)2 bpy, 4,4 -dimethoxy-2,2 -bipyridine; AgOTf, silver trifluoromethanesulfonate; TlOTf, thallium trifluoromethanesulfonate; LLCT, ligand-to-ligand charge transfer; MLCT, metal-to-ligand charge transfer; pimMe, 1-methyl-2-(2 -pyridyl)imidazole; pimPh, 1-phenyl-2-(2 -pyridyl)imidazole; THF, tetrahydrofuran. ∗ Corresponding author. Tel.: +852 3442 6958; fax: +852 3442 0522. E-mail address: [email protected] (C.-C. Ko). 0010-8545/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2012.01.006

Following the first report of the luminescence behaviour of rhenium(I) tricarbonyl diimine complexes by Wrighton and Morse in 1974 [1], quite a number of luminescent rhenium(I) tricarbonyl ␣,␣ -diimine complexes, [Re(CO)3 (N–N)(La )]n+ (n = 0 or 1; La : axial ligand), showing rich excited state properties associated with their long-lived triplet metal-to-ligand charge transfer (3 MLCT) excited state, were extensively reported [2–16]. The characteristic intense carbonyl (C≡O) stretching vibrations, which can be probed by time-resolved IR spectroscopy to follow the excited state dynamics [17–19], have made this class of complexes unique amongst other polypyridyl transition metal complexes with d6 metal centers,

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Scheme 1. Synthetic routes and reactivity of di(isocyano) rhenium complexes. Reproduced with permission from Ref. [47].

which also exhibit long-lived MLCT phosphorescence. Through elegant design together with the time-resolved IR spectroscopy, this class of complexes can be used as protein labels to study the electron transfer reactions and the structural dynamics in proteins [20–23]. With a suitable design and functionalization of the diimine or the ancillary ligands, these complexes can serve as photosensitizers [24–27], luminescent probes and sensors [28–33] as well as materials for electroluminescent devices and photovoltaic devices [34–37]. However, when the ligands are functionalized for specific applications, further modification and fine-tuning of the physical and excited state properties are challenging since carbonyl coligands cannot be functionalized and are difficult to be substituted with other ligands, though there has been some success in replacement of the carbonyl ligand with phosphine ligands [38–40]. As a result, multi-step organic synthesis on the diimine or the ancillary ligand is required to tune the properties of the complex and maintain the functionality of the ligands. In this context, we have designed and synthesized several new classes of readily tunable rhenium(I) diimine luminophores through replacement of the carbonyl ligands with isocyanide ligands. One of the advantages of using isocyanide ligands over carbonyl ligand in the design of metal complexes is that the physical, steric and electronic properties of metal isocyanide complexes can be effectively tuned through modification of the substituent on the nitrogen atom. Such flexibility has been demonstrated to be useful and beneficial in the design and study of transition metal isocyanide complexes for the application in catalysis [41–43]. As isocyanides (R–N≡C:) are isoelectronic to carbonyl ligand (O≡C:), their coordination abilities are similar. Therefore, isocyanide analogues of many metal carbonyl complexes can be prepared, even though their synthetic routes are generally different [44,45]. On the other hand, metal isocyanide complexes also exhibit strong and characteristic C≡N stretching vibrations [44,45], which also allow the study of the excited state dynamics with time-resolved IR spectroscopy. Thus, they are ideal ligands that can be used in replacement of the carbonyl ligands to provide the tunable feature for the rhenium(I) diimine luminophores and can concurrently retain the feasibility of probing the complexes with IR spectroscopy so that the excited state dynamics could also be studied and followed by time-resolved IR spectroscopy. In this review article, our recent work on the design, synthesis, characterization and photophysical properties of several

new classes of readily tunable isocyano rhenium(I) diimine luminophores [46–49] will be summarized. 2. Synthetic methodology and characterization All isocyanide ligands in this review have been prepared by the dehydration of corresponding formamide using POCl3 according to the method developed by Ugi et al. [50]. 2.1. Di(isocyano) dicarbonyl rhenium diimine complexes [Re(CO)2 (CNR)2 (N–N)]+ [47] Cis,cis-[Re(CO)2 (CNR)2 (N–N)]+ can be obtained by the ligand substitution reactions of fac-[Re(CO)3 (CNR)2 (MeCN)]+ with an excess of diimine ligands (N–N) in refluxing dioxane or toluene (Scheme 1a). However, the yields of the target di(isocyano) dicarbonyl diimine complexes are very low and difficult to be separated from the mixture of the major product, fac[Re(CO)3 (CNR)(N–N)]+ . In order to enhance the yield of the target di(isocyano) dicarbonyl diimine complexes, selective carbonyl ligand substitution based on photo-substitution reactions of fac[Re(CO)3 (CNR)2 Br] with an excess amount of diimine ligand in benzene solution (Scheme 1b) have been developed. These photosubstitution reactions are highly selective, versatile and give cis,cis-[Re(CO)2 (CNR)2 (N–N)]Br as the only major product. The Xray crystal structure of one of these complexes (3) has also been determined and the perspective drawing of the complex cation is shown in Fig. 1. The high selectivity of the ligand substitution reaction was also revealed in the in situ IR spectroscopic study in the reaction of fac-[Re(CO)3 (CNC6 H4 n Bu-4)2 Br] and t Bu2 bpy in [D6 ]benzene solution. Fig. 2 shows the IR spectra of the mixture of 15 mM fac-[Re(CO)3 (CNC6 H4 n Bu-4)2 Br] with 60 mM t Bu2 bpy in [D6 ]benzene solution in the range of 1890–2200 cm−1 recorded at different times after photoexcitation at 254 nm and the inset shows the absorption-time profiles at selected wavenumbers. Upon UV-excitation, three C≡O and two C≡N stretching vibrations of the carbonyl and isocyanide ligands in the precursor complex, fac-[Re(CO)3 (CNC6 H4 n Bu-4)2 Br], decreased in intensity and concomitant growth of two new C≡O and two new C≡N stretching vibrations of the final product, cis,cis[Re(CO)2 (CNC6 H4 n Bu-4)2 (t Bu2 bpy)]Br, were observed. Compared

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Fig. 1. Perspective drawing of the complex cation of 3 with atomic numbering. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 30% probability level. Reproduced with permission from Ref. [47].

to the precursor complex, the new C≡O and C≡N stretching vibrations were shifted to lower stretching frequencies, which is consistent with the better ␲-back-bonding interactions between the rhenium metal center and the unsubstituted carbonyl and isocyanide ligands as a stronger ␲-accepting carbonyl ligand was replaced by the diimine ligand. The time constants obtained from the single exponential fittings of the decay and the growth of the IR stretching vibrations of the starting precursor complex and the final product, respectively, are very similar (Fig. 2, inset). The close resemblance of these time constants together with the fairly well-defined isosbestic points in the IR absorption spectral changes during the course of the reaction are suggestive of a clean conversion from the starting precursor complex to the final product. 2.2. Tri(isocyano) carbonyl rhenium diimine complexes [Re(CO)(CNR)3 (N–N)]+ [48] With the successful photochemical ligand substitution reactions used in the preparation of cis,cis-[Re(CO)2 (CNR)2 (N–N)]+ , a similar synthetic strategy for the syntheses of tri(isocyano)

0.12

2.3. Tetra(isocyano) rhenium diimine complexes [Re(CNR)4 (N–N)]+ [46,49] Apart from the carbonyl containing isocyano rhenium(I) diimine complexes, the tetra(isocyano) rhenium(I) diimine complexes have also been prepared. These complexes were synthesized from K2 [ReI6 ] using a completely different synthetic approach. By the substitution reactions of [Re(CNR)5 I] [51] with an excess amount of diimine ligands (N–N) in the presence of TlOTf in refluxing 1,4dioxane for 2 days afforded the target tetra(isocyano) rhenium(I) diimine complexes (Scheme 4). The X-ray crystal structures of 21, 24, 27 and 32 have been studied and the perspective views of their cations are depicted in Fig. 4.

0.06

A

Absorbance

0.09

analogous complexes from the photo-substitution reactions of fac-[Re(CO)3 (CNR)3 ]OTf and mer-[Re(CO)2 (CNR)3 Br] with different diimine ligands has also been attempted. For the syntheses of fac-[Re(CO)3 (CNR)3 ]OTf (R = C6 H5 , 4-BrC6 H4 or 4-ClC6 H4 ), the photo-substitution reactions are highly stereoselective with the target complexes, fac-[Re(CO)(CNR)3 (N–N)]OTf, being the only major product (Scheme 2). However, in the preparation of other analogous complexes containing electron-releasing and/or bulky substituents on the isocyanide ligands or diimine ligands, the photo-substitution reactions led to the formation of isomeric mixture of both facial and meridional conformations (Scheme 2). As the fac- and mer-isomers of [Re(CO)(CNR)3 (N–N)]+ are difficult to separate by column chromatography, selective synthetic routes for the fac- and mer-isomers based on the trimethylamine N-oxide (Me3 NO) mediated oxidative decarbonylation reaction and the subsequent ligand substitution reactions (Scheme 2) have been developed. The X-ray crystal structures of complexes 10, 12–15, 17 and 18 have been studied and the perspective drawings of the complex cations of selected complexes are shown in Fig. 3. Due to the strong ␲-accepting ability associated with isocyanide ligands, they can compete with other ligands, in particular the trans-ligand, for ␲-back-bonding interaction with the metal center. As a result, the carbonyl ligand trans to isocyanide ligand is relatively more electrophilic compared to the one trans to diimine ligand and therefore it is selectively replaced in the Me3 NO mediated carbonyl ligand substitution reaction. This reactivity is illustrated in the preparation of cis,cis-[Re(CO)(Lx )(CNC6 H4 Cl4)2 (phen)]PF6 (Lx = MeCN or pyridine) (Scheme 3). With this reaction, it enhances the flexibility in functionalization of these carbonyl containing isocyano rhenium(I) diimine luminophores with different types of ligands.

0.04

hν

A1968-1960 A1998-1990

3. Photophysical properties

0.02

0

200 t / min

400

3.1. UV–vis absorption properties

0.06

0.03

0.00 2200

2100

2000

1900 −1

Wavenumber / cm

Fig. 2. In situ IR spectra of the reaction mixture of 15 mM fac-[Re(CO)3 (CNC6 H4 n Bu4)2 Br] with 60 mM t Bu2 bpy in [D6 ]benzene solution in the wavenumber range of 1890–2200 cm−1 recorded at various times: 0, 20, 40, 60, 80, 120, 140, 160, 180, 240, 300, 360 and 420 min after photoexcitation at 254 nm. The insets show the absorption spectral changes at selected wavenumbers. Reproduced with permission from Ref. [47].

The electronic absorption data of all these complexes in dichloromethane solutions were summarized in Table 1. All complexes display intense absorption at ca. 230–350 nm with molar absorption coefficients of the order of 104 dm3 mol−1 cm−1 , corresponding to the spin allowed intraligand (IL) ␲ → ␲* transitions of the isocyanide and diimine ligands. In addition, several moderately intense absorption bands or shoulders in the lower energy region of 360–492 nm are also observed in the absorption spectra of these complexes. With reference to the previous spectroscopic studies of homoleptic metal isocyanide complexes [52–54] and tricarbonyl Re(I) diimine complex systems [1–4,11–16], this moderately intense absorption was ascribed to MLCT [d␲(Re) → ␲*(CNR)] and [d␲(Re) → ␲*(N–N)] transitions and probably mixed with the

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Scheme 2. Synthetic routes and reactivity of tri(isocyano) rhenium complexes. Reproduced with permission from Ref. [48].

Fig. 3. Perspective drawings of the complex cations of (a) 10, (b) 18, (c) 15 and (d) 14 with atomic numbering. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 30% probability level. Reproduced with permission from Ref. [48].

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isocyanide ligands. These observed absorption energy dependences result from the modification of the orbital energy level of d␲(Re) through the rhenium-isocyanide [d␲(Re)–␲*(CNR)] interactions and orbital energy level of ␲*(N–N) by substituents with different electronic features as well as the extent of ␲-conjugation. Therefore they are consistent with the assignment of an MLCT [d␲(Re) → ␲*(N–N)] transition. Although the ligand-to-ligand charge transfer (LLCT) [␲(CNR) → ␲*(N–N)] transitions are also expected to show similar energy trends, this assignment for the lowest-energy absorption is less likely because these absorption bands shift significantly in the order of [Re(CO)2 (CNR)2 (N–N)]+ > [Re(CO)(CNR)3 (N–N)]+ > [Re(CNR)4 (N–N)]+ for complexes with isocyanide and diimine ligands of similar electronic nature (Fig. 5). This trend can be rationalized by the higher basicity and the weaker ␲-accepting ability of the isocyanide ligands compared to the carbonyl ligand, and thus the d␲(Re) orbital would become higher-lying in energy when a carbonyl ligand is replaced by an isocyanide ligand.

Scheme 3. Synthetic routes for cis,cis-[Re(CO)(Lx )(CNC6 H4 Cl-4)2 (phen)]PF6 . Reproduced with permission from Ref. [48].

ligand-to-ligand charge transfer (LLCT) [␲(CNR) → ␲*(N–N)] transition. As the ␲* orbital of aryl isocyanides was higher-lying in energy compared to those of diimine ligands, the lowest energy MLCT absorption bands are assigned to the MLCT [d␲(Re) → ␲*(N–N)] transitions. These absorption bands show strong energy dependencies on the ␲-accepting abilities of both the isocyanide and diimine ligands along each series of the complexes containing same number of isocyanide and carbonyl ligands. In general, the abs is in line with the ␲-accepting ability of diimine ligand and in the reverse order of the ␲-accepting ability of the isocyanide ligand. These trends can be exemplified by the order of 11 (430 nm) < 10 (443 nm) for [Re(CO)(CNC6 H5 )3 (R2 phen)]+ with different substituents on the phenanthroline ligand and 13 (411 nm) < 12 (418 nm) < 11 (430 nm) < 16 (437 nm) ≈ 17 (436 nm) for [Re(CO)(CNR)3 (phen)]+ with different substituents on the

I

Re

I I

I I

All complexes display green to red phosphorescence upon excitation at  > 350 nm in CH2 Cl2 solution with emission maxima in the range of 530–738 nm (Table 1). These emissions not only show energy dependence on the electronic nature of the isocyanide and diimine ligands but also vary with the isomeric configuration of the complex system similar to that for the lowest-energy absorption bands. Based on these energy dependences, as well as previous spectroscopic and computation study of related rhenium diimine complexes [1–4,11–16,55], this emission was assigned as phosphorescence derived from the metal-to-ligand charge transfer (MLCT) [d␲(Re) → ␲*(N–N)] excited state origin mixed with some LLCT character [␲(CNR) → ␲*(N–N)]. The sub-microsecond emission lifetimes of these complexes are consistent with the

R N C

2−

I

3.2. Emission properties

RNC

RNC

NH2NH2

RNC

CNR

Re

CNR I

1. TlOTf RNC 2.

N

N

CNR

Re

RNC

PF6−

N

3. NH4PF6 MeO

+

R N C

N X1

OMe N

N

N =

N N (bpy) X2

N N (tBu2bpy)

N N [(MeO)2bpy] Br

N N (phen)

X3

X2 = F, Cl, Br, I

R= X3 21 22 23 24 25 26 27 28 29

N N (X1 = Me; pimMe) (X1 = Ph; pimPh)

R = C6H4I-4; N-N = phen R = C6H2Br-4-Me2-2,6; N-N = phen R = C6H2Br3-2,4,6; N-N = phen R = C6H2Cl3-2,4,6; N-N = phen R = C6H2Cl2-2,4-OMe-6; N-N = phen R = C6H4I-4; N-N = bpy R = C6H4Br-4; N-N = bpy R = C6H2Br-4-Me2-2,6; N-N = (MeO)2bpy R = C6H2Cl3-2,4,6; N-N = (MeO)2bpy

X3 30 31 32 33 34 35 36 37 38

X3 = Cl, Br

R = C6H4Cl-4; N-N = pimPh R = C6H4I-4; N-N = pimMe R = C6H4Cl-4; N-N = pimMe R = C6H2Br-4-Me2-2,6; N-N = pimMe R = C6H4Cl-4; N-N = (MeO)2bpy R = C6H4Cl-4; N-N = tBu2bpy R = C6H4Cl-4; N-N = bpy R = C6H4F-4; N-N = bpy R = C6H3Me2-2,6; N-N = bpy

Scheme 4. Synthetic routes and reactivity of tetra(isocyano) rhenium complexes. Reproduced with permission from Ref. [49].

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Fig. 4. Perspective drawings of the complex cations of (a) 21, (b) 24, (c) 27 and (d) 32 with atomic numbering. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 30% probability level. Reproduced with permission from Ref. [49].

triplet emissive excited state origin. Although the emission lifetimes ( o ) and the luminescence quantum yields (em ) for different series of complexes are significantly different (Table 1), a careful investigation on the variation of the rate of non-radiative decay (knr ), calculated from the  o and em , and the emission energy (Eem ) reveals that all phenanthroline complexes follow the energy gap law [4,56,57] as reflected by the linear relationship (R = 0.97) in the plot of ln(knr ) and emission energy (Eem ) (Fig. 6) with slope of −10.9 eV−1 , which is similar in magnitude compared to those reported for other MLCT emitters [58–62]. This

correlation also suggests that the emissive excited states of these phenanthroline complexes have similar vibrational and electronic components. With these complex systems, the energy of the phosphorescence of the rhenium(I) diimine luminophores can be systematically tuned and tailored by the following strategies: (1) choosing of an appropriate complex system as the

17 +

16

+

+

[Re(CNC6H4Cl)3(CO)(phen)] (13)

60

+

[Re(CNC6H4I)4(phen)] (21)

[Re(CO)2(CNR)2(phen)]

17 21 20

[Re(CNC6H4Cl)2(CO)2(phen)] (2)

ln knr

ε / 103 cm−1 mol−1 dm3

19

t

[Re(CN Bu)2(CO)2(phen)] (7)

80

22

15

16

[Re(CO)(CNR)3(phen)] + 11 12 [Re(CNR)4(phen)] 18 13 23 25 24

+

+

14

40

7

13 20

5 2 4 3

12 0 300

400

500

15

600

), 7 (—), 13 (

17 3

Eem / 10 cm

Wavelength / nm Fig. 5. Overlaid UV–vis absorption spectra of 2 ( ( ) in CH2 Cl2 solutions at 298 K.

16

) and 21

18

1

19

-1

Fig. 6. Plot of ln knr versus Eem (298 K, CH2 Cl2 ) for phenanthroline complexes with linear least-squares fit.

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Table 1 UV–vis absorption and emission data of 1–38 in dichloromethane solution at 298 K [46–49].

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 a

Emission em /nm ( o /␮s)

em a ×103

Absorption abs /nm (ε/M−1 cm−1 )

530 (3.37) 553 (2.97) 551 (3.17) 558 (3.29) 562 (2.88) 620 (0.002) 576 (1.72) 573 (5.45) 556 (0.90) 676 (0.031) 640 (0.075) 625 (0.090) 621 (0.146) 607 (0.0992) 618 (0.055) 650 (0.082) 674 (0.046) 625 (0.128) 690 (0.050) 665 (0.068) 674 (0.061) 686 (0.039) 616 (0.241) 614 (0.275) 629 (0.187) 684 (0.013) 686 (0.013) 683 (0.005) 612 (0.039) 645 (0.099) 631(0.173) 632 (0.159) 648 (0.134) 681 (0.007) 671 (0.020) 690 (0.012) 698 (0.009) 738 (0.005)

370 340 350 280 310 6 80 310 230 5.09 19.4 51.9 36.7 26.5 24.4 17.4 5.14 26.1 4.43 5.57 4.6 1.7 43.2 50.3 25.4 0.40 0.50 0.49 3.1 19.1 27.9 22.6 13.8 0.57 2.9 1.1 0.62 0.16

255 (43,770), 273 (49,950), 316 (25,960), 366 (14,550), 413 sh (3980) 272 (68,040), 325 (20,370), 355 (14,210), 426 sh (4100) 272 (73,380), 324 (20,370), 357 (14,330), 427 sh (3800) 275 (65,110), 285 (65,750), 310 (47,800), 360 (20,700), 427 sh (5230) 269 (70,930), 320 (14,190), 355 (10,880), 428 sh (3350) 272 (68,040), 325 (20,370), 355 (14,210), 460 sh (3780) 262 (39,430), 288 (16,610), 369 (5780), 430 sh (3050) 279 (48,200), 350 (10,460), 440 sh (3240) 246 (39,550), 275 (56,630), 352 (10,400), 425 sh (2970) 240 (63,490), 276 (87,500), 290 sh (53,810), 313 sh (33,140), 330 sh (22,750), 386 (11,850), 443 sh (9215) 266 (79,960), 291 (49,440), 338 (21,535), 377 (14,130), 430 sh (8740) 266 (75,140), 293 sh (50,905), 344 (28,235), 384 (18,080), 418 sh (10,590) 267 (75,080), 293 (53,745), 386 sh (19,400), 411 sh (11,355) 240 (56,570), 288 (67,930), 310 (67,315), 348 sh (32,310), 394 sh (19,710) 241 (62,990), 285 (78,290), 307 sh (54,265), 344 (31,995), 392 (18,355) 267 (72,140), 293 (45,750), 336 (20,770), 384 (12,780), 437 (8230) 267 (92,200), 293 (62,995), 322 sh (30,000), 382 (15,405), 436 (9575) 268 (76,785), 294 (59,650), 342 sh (24,350), 385 (17,715) 271 (62,820), 295 (35,760), 324 (30,360), 374 (16,390), 453 (6620) 269 (55,400), 292 (33,715), 319 (25,610), 371 (14,615), 435 (7520) 269 (72,455), 290 (41,605), 331 (49,520), 385 (28,940), 410 sh (17,080), 465 sh (7890) 269 (80,765), 293 (53,240), 327 (68,020), 382 (33,410), 479 (8890) 229 (92,790), 267 (89,820), 292 (45,950), 344 (63,475), 381 sh (45,495), 393 sh (42,910), 419 sh (25,665) 229 (90,810), 266 (88,270), 293 (45,010), 342 (62,495), 390 sh (43,080), 419 sh (26,195) 267 (85,705), 291 (43,810), 341 (58,645), 376 sh (43,965), 430 sh (18,925) 254 (58,780), 295 (72,400), 334 (67,420), 383 (38,020), 406 sh (21,155), 450 sh (9440) 244 (60,835), 296 (69,680), 331 (60,670), 380 (32,525), 412 sh (14,780), 451 (8205) 246 (72,160), 288 (56,440), 335 (74,515), 376 (41,255), 418 sh (16,990) 262 (55,525), 283 (41,925), 349 (51,155), 377 sh (37,850), 402 (30,840) 242 (59,790), 307 (68,010), 334 (58,405), 380 sh (30,495), 414 sh (18,365) 259 (60,170), 307 (64,100), 338 (61,540), 386 (37,605), 411 sh (23,690) 240 (54,710), 306 (61,810), 333 (54,475), 385 (30,105), 408 sh (19,840) 248 (54,885), 309 (55,005), 333 (53,460), 383 (28,350), 408 sh (17,500) 231 (72,610), 252 (52,390), 283 (53,615), 332 (59,270), 375 (33,105), 412 sh (16,160) 239 (55,965), 294 (66,050), 331 (57,320), 365 (33,980), 412 sh (15,020) 230 (65,215), 296 (78,970), 328 (66,405), 366(38,610), 408 sh (17,450), 456 sh (9095) 235 (38,200), 296 (52,585), 315 (43,390), 358 (22,345), 400 sh (9430), 471 (5520) 242 (53,940), 298 (69,555), 322 (52,095), 360 (29,910), 410 sh (9830), 492 (6525)

Luminescence quantum yield with excitation at 436 nm.

emission energy follows the order of [Re(CO)2 (CNR)2 (N–N)]+ > [Re(CO)(CNR)3 (N–N)]+ > [Re(CNR)4 (N–N)]+ for complexes with isocyanide and diimine ligands of similar ␲-accepting abilities (Fig. 7a); (2) incorporation of diimine ligand with different ␲*(N–N) orbital energies either by modifying the extent of ␲-conjugation or changing the electronic effect of the substituents on the diimine ligand; (3) changing the electronic nature of the isocyanide ligands so as to perturb the rhenium-isocyanides [d␲(Re)–␲*(CNR)] interactions and the resulting d␲(Re) orbital energy (Fig. 7b); (4) changing the structural isomeric configurations of the complex system as illustrated by the difference in the emission energy between the fac- and mer-isomers of [Re(CO)(CNR)3 (N–N)]+ (Fig. 7c). 3.3. Emission solvatochromic behaviour of [Re(CNR)4 (N–N)]+ In our early communication [46], the highly solvent sensitive emission of 34 was initially revealed. To elucidate the structure–relationship of the emission solvatochromic behaviour, the emission of a series of [Re(CNR)4 (N–N)]PF6 (21–38) with isocyanide and diimine ligands of diverse electronic and steric natures in wide variety of solvents, such as benzene, dioxane, THF, CHCl3 , CH2 Cl2 , acetone, MeCN, ethanol and methanol, have been investigated[49]. Different emission solvatochromic behaviour and degree of solvent dependence were observed for complexes having different types of diimine and isocyanide ligands (Fig. 8). For complexes with symmetrical diimine ligands, i.e. the substituted 2,2 -bipyridines and phenanthroline, their emissions show a blue shift with decreasing solvent polarity from an acetonitrile solution to a benzene solution (Fig. 8a). Such a solvent dependence

was attributed to a more polar excited state, which is more stabilized in a polar solvent medium, compared to its ground state. Moreover, except for the emission in methanol and ethanol, their emission energies estimated from the em are linearly correlated with Dimroth’s solvent parameter [63,64] (Fig. 9). The deviation of the emission data in methanol and ethanol from the correlation can be ascribed to the significant hydrogen-bonding interaction, which is absent in these complexes as they do not have any functional moiety capable of forming hydrogen bonding, between the protic solvents and pyridinium phenoxidedye used in the derivation of Dimroth’s solvent parameter [63,64]. By comparing the slope of the plots (Fig. 9), the degree of solvent dependence of the emission of these complexes can be evaluated. This varies with the nature of diimine ligand with the extent in the order of (MeO)2 bpy ≈ t Bu2 bpy  bpy > phen. Such a decrease of the solvatochromic shifts with the increasing ␲-acceptor strength of the diimine ligand can be explained by the decrease in charge transfer character due to the stronger mixing of d␲ orbital of metal center and ␲* orbital of diimine ligand by better ␲-back-bonding interaction and is commonly reported in the study of the solvent effects on the MLCT emissions of related complexes [65–68]. Apart from the electronic effect of the diimine ligand, the emission solvatochromic behaviour of these complexes is also remarkably affected by the steric properties of the isocyanide ligands. For complexes with 2,4,6-trisubstituted phenylisocyanide ligands, their emissions are much less solvent sensitive compared to those with 4-monosubstituted phenylisocyanide ligands as the excited portions of the complexes are better shielded by more sterically bulky isocyanide ligands. Similar shielding effects of the excited state

C.-C. Ko et al. / Coordination Chemistry Reviews 256 (2012) 1546–1555

Normalized Emission Intensity

11 12 13 16 17

(b)

18 −1

Benzene Dioxane THF CHCl3 CH2Cl2 Acetone

3

Emission Energy / 10 cm

2 13 21

(a)

1553

26 28 29 MeCN

16

EtOH

36 37 38 MeOH

0.95

0.90 0.95 0.94 0.97 0.92

14

36

40

44

48

52

56

Dimroth's Solvent Parameter (c)

14 18 11 15

500

600

700

800

Wavelength / nm Fig. 7. Normalized emission spectra of (a) [Re(CO)4−n (CNR)n (phen)]+ (n = 2, 3, CNR = CNC6 H4 Cl-4; n = 4, CNR = CNC6 H4 I-4), (b) fac-[Re(CO)(CNR)3 (phen)]PF6 with different isocyanide ligands (CNR = CNC6 H5 , CNC6 H4 Cl-4, CNC6 H4 Br-4, CNC6 H3 (i Pr)2 –2,6, CNC6 H4 OMe-4) and (c) fac- (solid lines) and mer- (dotted lines) isomers of [Re(CO)(CNC6 H5 )3 (phen)]PF6 (red lines) and [Re(CO)(CNC6 H4 Cl4)3 (t Bu2 bpy)]PF6 (blue lines) in CH2 Cl2 solution at 298 K.

Fig. 9. A plot of emission energy of 26, 28, 29, 36–38 in different solvents versus Dimroth’s solvent parameter and their linear least-squares fits (—) excluding the data points of methanol and ethanol. Reproduced with permission from Ref. [49].

ligands are almost solvent insensitive and do not show obvious emission energy dependence on the polarity of the solvent media (Fig. 8b). The complexes 30–32 with unsymmetrical substituted pyridine imidazole ligands show a different solvent dependence with the lowest energy emission in non-polar solvents, such as benzene and dioxane, compared to other polar solvents (Fig. 8c). This may be due to the significant variations of the dipole moments of the complexes in the ground state and the excited state. However, these emissions do not show any correlation with the solvent parameters. 4. Transient absorption spectroscopy

Reproduced with permission from Ref. [48].

against environmental perturbation have also been reported in the emission solvatochromic studies of related MLCT luminophores [69–71]. As a consequence, the emissions of phenanthroline complexes 22–25 with different 2,4,6-trisubstituted phenylisocyanide

Normalize Emission Intensity

(a)

Benzene Dioxane THF CHCl3

Solvent Polarity

(b)

CH2Cl2 Acetone MeCN MeOH

(c)

560

630

700

770

Wavelength / nm Fig. 8. Normalized uncorrected emission spectra of (a) 29, (b) 24 and (c) 30 in different solvents at 298 K. Reproduced with permission from Ref. [49].

As the emission lifetimes of these complexes are in the submicrosecond range, the absorption properties of the emissive excited states of the phenanthroline complexes have also been investigated by nanosecond transient absorption spectroscopy. Fig. 10 displays the transient absorption difference spectra of [Re(CO)2 (CNt Bu)2 (phen)]PF6 (7) after 355-nm nanosecond laser excitation. Two absorption features peaking at ca. 300–310 nm and 480–520 nm, respectively, were observed. However, the transient absorption signals at  > 560 nm cannot be obtained as they overlapped significantly with the photoluminescence of the complex. The decays of these transient absorption features follow the first order kinetics and the lifetimes are consistent with the luminescence intensity decays. The close agreement of these decay profiles with the phosphorescence decay is suggestive of their identical origin. In view of the close resemblance of these absorption features with the transient absorption spectra of related rhenium(I) phenanthroline complexes [Re(CO)3 (phen)(L)] [3,72–74], this transient absorption is also attributed to the absorption of the triplet MLCT [d␲(M) → ␲*(phen)] state. The assignments of the excited-state triplet–triplet transitions have recently been discussed in detail and investigated by time-dependent DFT calculations [72–74]. In contrast to 7, the transient absorption difference spectra of substituted phenylisocyano analogous complexes (1, 2 and 5) and [Re(CNC6 H2 Cl3 )4 (phen)]+ (24) show weaker absorption at ca. 300–310 nm with strong ground state bleaching at 310–400 nm in their difference spectra (Fig. 11), where they showed much stronger ground state absorption than 7 (Fig. 5). This ground state bleaching is likely associated with the MLCT [d␲(Re) → ␲*(CNR)], LLCT [␲(CNR) → ␲*(phen)] and MLCT [d␲(Re) → ␲*(phen)] transitions. The weaker ground state absorption and the absence of the ground state bleaching for 7 in the region of 310–400 nm is due to

C.-C. Ko et al. / Coordination Chemistry Reviews 256 (2012) 1546–1555

ΔOD311 nm

1554

(a) ΔOD / 0.01

4 2

τ = 89 μs

5. Concluding remarks

0

2000 4000

t / ns

0 -2

I605 nm

Emission Intensity

(b)

0

τ = 90 μs

3000

6000

t / ns 300

400

500

600

700

Wavelength / nm Fig. 10. (a) Transient difference spectra of 7 in MeCN solution at 298 K obtained after 355-nm nanosecond laser excitation at different time delays: 0, 0.2, 0.5, 0.8, 1.2, 1.6, 2.0, 2.5, 3.0, and 4.0 ␮s. (b) Emission spectrum of 7 in MeCN solution. The insets show (a) the absorption–time profile at 311 nm and (b) luminescence intensity decay trace at 605 nm after 355-nm nanosecond laser excitation and their first order exponential fit (red line).

the much higher energies for the MLCT [d␲(Re) → ␲*(t BuNC)] and LLCT [␲(t BuNC) → ␲*(phen)] transitions as tert-butyl isocyanide ligand possesses much higher-lying ␲* and lower-lying ␲ orbitals compared to the isocyanide ligands with ␲-conjugating phenyl substituents [53]. In light of the close similarity of the absorption in the visible region and the weak absorption at ca. 300 nm for these complexes and 7, the transient absorption for these complexes

4

(a)

2

7

2 (b) 0 -2 -4

1

2 (c) 1 0 -1

2

t

BuNC

Cl3C6H2NC

ClC6H4NC

5 2 (d) 1 0 Me2C6H2NC -1 6 (e) 24 0 -6 + -12 Cl C H NC [Re(phen)(CNR) ] 4 3 6 2 300

This review describes our recent work on the design and study of several new classes of tunable luminescent isocyano rhenium(I) diimine complexes. Different synthetic methodologies have been developed for the preparation of the rhenium(I) diimine complexes containing two, three and four isocyanide ligands. Most of these complexes are robust and photo-stable. The X-ray crystal structures, photophysical and transient absorption properties of these complexes have been investigated. Further functionalization of the carbonyl containing isocyano rhenium(I) diimine complexes with different types of ligand through the Me3 NO mediated carbonyl ligand substitution reaction have also been developed. Detailed studies revealed that both the physical and excited state properties of these complexes could be readily tuned through varying the substituents on the isocyanide ligands. In addition, this should open up the possibility in the extension of the rhenium bipyridine luminophores towards the construction of highly functionalized molecular systems of widely diverse solubility and property by simply altering the substituent groups on the isocyanide ligands or introducing different types of ancillary ligands. It is anticipated that with the flexibility, tunability and the rich excited state properties associated with these complexes, they are excellent building blocks for various applications such as photosensitizers, luminescent probes and sensors as well as materials for electroluminescent devices and solar energy conversion. Acknowledgements The work described in this paper was supported by a RGC GRF grant from the Research Grants Council of the Hong Kong (Project No. CityU 101510) and a grant from City University of Hong Kong (Project No. 7002688). The flash photolysis system was supported by the Special Equipment Grant from the University Grants Committee of the Hong Kong (SEG CityU02). A.W.-Y. Cheung, L.T.-L. Lo, J.W.-K. Siu and C.-O. Ng acknowledge the receipt of a Postgraduate Studentship from the City University of Hong Kong. A.W.-Y. Cheung and L.T.-L. Lo also acknowledge the receipt of a Research Tuition Scholarship administrated by City University of Hong Kong. References

Emission Intensity

Δ OD

0 -2

is also tentatively assigned to the absorption of the triplet MLCT excited state.

400

500

600

[1] [2] [3] [4] [5] [6] [7] [8] [9] 10 [11]

700

Wavelength / nm Fig. 11. Overlaid transient difference (solid line) and emission spectra (dashed line) of dicarbonyl diisocyano rhenium(I) phenanthroline complexes [(a) 7, (b) 1, (c) 2 and (d) 5] and (e) tetraisocyano rhenium(I) phenanthroline complex (24) in MeCN solution at 298 K.

[12] [13] [14] [15] [16] [17]

[18]

M.S. Wrighton, D.L. Morse, J. Am. Chem. Soc. 96 (1974) 998. J.C. Luong, L. Nadjo, M.S. Wrighton, J. Am. Chem. Soc. 100 (1978) 5790. K. Kalyanasundaram, J. Chem. Soc., Faraday Trans. 282 (1986) 2401. L.A. Worl, R. Duesing, P. Chen, L.D. Ciana, T.J. Meyer, J. Chem. Soc., Dalton Trans. (1991) 849. L. Sacksteder, M. Lee, J.N. Demas, B.A. DeGraff, J. Am. Chem. Soc. 115 (1993) 8230. H. Hori, F.P.A. Johnson, K. Koike, K. Takeuchi, T. Ibusuki, O. Ishitani, J. Chem. Soc., Dalton Trans. (1997) 1019. L. Sacksteder, A.P. Zipp, E.A. Brown, J. Streich, J.N. Denas, B.A. DeGraff, Inorg. Chem. 29 (1990) 4335. V.W.-W. Yam, V.C.-Y. Lau, K.-K. Cheung, Organometallics 14 (1995) 2749. T.A. Oriskovich, P.S. White, H.H. Thorp, Inorg. Chem. 34 (1995) 1629. W.B. Connick, A.J. Di Bilio, M.G. Hill, J.R. Winkler, H.B. Gray, Inorg. Chim. Acta 240 (1995) 169. R.V. Slone, D.I. Yoon, R.M. Calhoun, J.T. Hupp, J. Am. Chem. Soc. 117 (1995) 11813. S.-S. Sun, A.J. Lees, J. Am. Chem. Soc. 122 (2000) 8956. V.W.-W. Yam, Chem. Commun. (2001) 789. J.M. Villegas, S.R. Stoyanov, W. Huang, D.P. Rillema, Dalton Trans. (2005) 1042. R.A. Kirgan, B.P. Sullivan, D.P. Rillema, Top. Curr. Chem. 281 (2007) 45. A. Kumar, S.-S. Sun, A.-J. Lees, Top. Organomet. Chem. 29 (2010) 1. D.R. Gamelin, M.W. George, P. Glyn, F.W. Grevels, F.P.A. Johnson, W. Klotzbucher, S.L. Morrison, G. Russell, K. Schaffner, J.J. Turner, Inorg. Chem. 33 (1994) 3246. D.M. Dattelbaum, K.M. Omberg, J.R. Schoonover, R.L. Martin, T.J. Meyer, Inorg. Chem. 41 (2002) 6071.

C.-C. Ko et al. / Coordination Chemistry Reviews 256 (2012) 1546–1555 [19] D.J. Liard, M. Busby, P. Matousek, M. Towrie, A. Vlˇcek Jr., J. Phys. Chem. A 108 (2004) 2363. [20] A.J. Di Bilio, B.R. Crane, W.A. Wehbi, C.N. Kiser, M.M. Abu-Omar, R.M. Carlos, J.H. Richards, J.R. Winkler, H.B. Gray, J. Am. Chem. Soc. 123 (2001) 3181. [21] B.R. Crane, A.J. Di Bilio, J.R. Winkler, H.B. Gray, J. Am. Chem. Soc. 123 (2001) 11623. [22] W. Belliston-Bittner, A.R. Dunn, Y.H. Le Nguyen, D.J. Stuehr, J.R. Winkler, H.B. Gray, J. Am. Chem. Soc. 127 (2005) 15907. [23] A.M. Blanco-Rodríguez, M. Busby, K. Ronayne, M. Towrie, C. Gr˘adinaru, J. Sud´ hamsu, J. Sykora, M. Hof, S. Záliˇs, A.J. Di Bilio, C.R. Crane, H.B. Gray, A. Vlˇcek Jr, J. Am. Chem. Soc. 131 (2009) 11788. [24] J. Hawecker, J.-M. Lehn, R. Ziessel, J. Chem. Soc., Chem. Commun. (1983) 536. [25] J.R. Shaw, R.T. Webb, R.H. Schmehl, J. Am. Chem. Soc. 112 (1990) 1117. [26] V.W.-W. Yam, C.-C. Ko, L.-X. Wu, K.M.-C. Wong, K.-K. Cheung, Organometallics 19 (2000) 1820. [27] C.-C. Ko, W.-M. Kwok, V.W.-W. Yam, D.-L. Phillips, Chem. Eur. J. 12 (2006) 5840. [28] D.B. MacQueen, K.S. Schanze, J. Am. Chem. Soc. 113 (1991) 6108. [29] D.I. Yoon, C.A. BergBrennan, H. Lu, J.T. Hupp, Inorg. Chem. 31 (1992) 3192. [30] L.H. Uppadine, J.E. Redman, S.W. Dent, M.G.B. Drew, P.D. Beer, Inorg. Chem. 40 (2001) 2860. [31] S.J.A. Pope, B.J. Coe, S. Faulkner, Chem. Commun. (2004) 1550. [32] A. Dirksen, C.J. Kleverlaan, J.N.H. Reek, L. De Cola, J. Phys. Chem. A 109 (2005) 5248. [33] K.K.-W. Lo, W.-K. Hui, C.-K. Chung, K.H.-K. Tsang, T.K.-M. Lee, C.-K. Li, J.S.-Y. Lau, D.C.-M. Ng, Coord. Chem. Rev. 250 (2006) 1724. [34] S.-T. Lam, N. Zhu, V.W.-W. Yam, Inorg. Chem. 48 (2009) 9664. [35] C.-W. Tse, K.K.-Y. Man, K.-W. Cheng, C.S.-K. Mak, W.-K. Chan, C.-T. Yip, Z.-T. Liu, A.B. Djurisic, Chem. Eur. J. 13 (2007) 328. [36] F. He, Y. Zhou, S. Liu, L. Tian, H. Xu, H. Zhang, B. Yang, Q. Dong, W. Tian, Y. Ma, J. Shen, Chem. Commun. (2008) 3912. [37] M. Mauro, E.Q. Procopio, Y. Sun, C.-H. Chien, D. Donghi, M. Panigati, P. Mercandelli, P. Mussini, G. D’Alfonso, L. De Cola, Adv. Funct. Mater. 19 (2009) 2607. [38] O. Ishitani, K. Kanai, Y. Yamada, K. Sakamoto, Chem. Commun. (2001) 1514. [39] S. Sato, T. Morimoto, O. Ishitani, Inorg. Chem. 46 (2007) 9051. [40] Y. Yamamoto, S. Sawa, Y. Funada, T. Morimoto, M. Falkenstrom, H. Miyasaka, S. Shishido, T. Ozeki, K. Koike, O. Ishitani, J. Am. Chem. Soc. 130 (2008) 14659. [41] B.M. Trost, C.A. Merlic, J. Am. Chem. Soc. 112 (1990) 9590. [42] S. Braune, U. Kazmaier, Angew. Chem., Int. Ed. 42 (2003) 306. [43] M. Suginome, T. Iwanami, Y. Ohmori, A. Matsumoto, Y. Ito, Chem. Eur. J. 11 (2005) 2954. [44] L. Malatesta, Prog. Inorg. Chem. 1 (1959) 284. [45] L. Weber, Angew. Chem., Int. Ed. 37 (1998) 1515. [46] C.-O. Ng, L.T.-L. Lo, S.-M. Ng, C.-C. Ko, N. Zhu, Inorg. Chem. 47 (2008) 7447.

1555

[47] C.-C. Ko, L.T.-L. Lo, C.-O. Ng, S.-M. Yiu, Chem. Eur. J. 16 (2010) 13773. [48] A.W.-Y. Cheung, L.T.-L. Lo, S.-M. Yiu, C.-C. Ko, Inorg. Chem. 50 (2011) 4798. [49] C.-C. Ko, J.W.-K. Siu, A.W.-Y. Cheung, S.-M. Yiu, Organometallics 30 (2011) 2701. [50] I. Ugi, U. Fetzer, U. Eholzer, H. Knupfer, K. Offermann, Angew. Chem., Int. Ed. Engl. 4 (1965) 472. [51] C.J. Cameron, D.E. Wigley, R.E. Wild, T.E. Wood, R.A. Walton, J. Organomet. Chem. 255 (1983) 345. [52] K.R. Mann, M. Cimolino, G.L. Geoffroy, G.S. Hammond, A.A. Orio, G. Albertin, H.B. Gray, Inorg. Chim. Acta 16 (1976) 97. [53] G.L. Geoffory, M.S. Wrighton, Organometallic Photochemistry, Academic Press, New York, 1980, pp. 259–276. [54] D.M. Roundhill, Photochemistry and Photophysics of Metal Complexes, Springer, New York, 1994, pp. 216–270. [55] S.R. Stoyanov, J.M. Villegas, A.J. Cruz, L.L. Lockyear, J.H. Reibenspies, D.P. Rillema, J. Chem. Theor. Comput. 1 (2005) 95. [56] K.F. Feed, J. Jortner, J. Chem. Phys. 52 (1970) 6272. [57] F.K. Fong, Theory of Molecular Relaxation, Wiley, New York, 1975. [58] J.V. Caspar, E.M. Kober, B.P. Sullivan, T.J. Meyer, J. Am. Chem. Soc. 104 (1982) 630. [59] E.M. Kober, J.V. Caspar, R.S. Lumpkin, T.J. Meyer, J. Phys. Chem. 90 (1986) 3122. [60] L. Sacksteder, A.P. Zipp, E.A. Brown, J. Streich, J.N. Demas, B.A. DeGraff, Inorg. Chem. 29 (1990) 4335. [61] R.N. Dominey, B. Hauser, J. Hubbard, J. Dunham, Inorg. Chem. 30 (1991) 4754. [62] S.D. Cummings, R. Eisenberg, J. Am. Chem. Soc. 118 (1996) 1949. [63] C. Reichardt, K. Dimroth, Fortschr. Chem. Forsch. 11 (1968) 1. [64] C. Reichardt, Angew. Chem., Int. Ed. 4 (1965) 29. [65] D.J. Stufkens, A. Vlˇcek Jr., Coord. Chem. Rev. 177 (1998) 127. [66] G. Knör, M. Leirer, A. Vogler, J. Organomet. Chem. 610 (2000) 16. [67] A. Vlˇcek Jr., Coord. Chem. Rev. 230 (2002) 225. [68] L. Rodríguez, M. Ferrer, O. Rossell, F.J.S. Duarte, A.G. Santos, J.C. Lima, J. Photochem. Photobiol. A: Chem. 204 (2009) 174. [69] E.R. Carraway, J.N. Demas, B.A. DeGraff, J.R. Bacon, Anal. Chem. 63 (1991) 337. [70] L. Sacksteder, E. Baralt, B.A. DeGraff, C.M. Lukehart, J.N. Demas, Inorg. Chem. 30 (1991) 2468. [71] W. Lu, D.A. Vicic, J.K. Barton, Inorg. Chem. 44 (2005) 7970. [72] A. El Nahhas, A. Cannizzo, F. van Mourik, A.M. Blanco-Rodríguez, S. Záliˇs, A. Vlˇcek Jr., M. Chergui, J. Phys. Chem. A 114 (2010) 6361. [73] A. El Nahhas, C. Consani, A.M. Blanco-Rodríguez, K.M. Lancaster, O. Braem, A. Cannizzo, M. Towrie, I.P. Clark, S. Záliˇs, M. Chergui, A. Vlˇcek Jr, Inorg. Chem. 50 (2011) 2932. [74] S. Záliˇs, C. Consani, A. El Nahhas, A. Cannizzo, M. Chergui, F. Hartl, A. Vlˇcek Jr, Inorg. Chim. Acta 374 (2011) 578.